The research, published in National Science Review, was led by Prof. Wei Xu of the Institute of Urban Environment and Prof. Ru-Jin Huang of the Institute of Earth Environment, both of the Chinese Academy of Sciences, together with Prof. Jurgita Ovadnevaite of the University of Galway. The team reports the first size-resolved observational constraint on the number size distribution of marine primary organic aerosols, achieved through long-term aerosol mass spectrometry, hygroscopicity measurements and machine-learning-based source apportionment. The measurements were carried out at the Mace Head Atmospheric Research Station on the west coast of Ireland, a location chosen for its frequent exposure to clean marine air masses arriving from the open North Atlantic, far from continental pollution sources that could confound the signal.

The reason this measurement matters so much lies in a fundamental distinction between mass and number. Marine primary organic aerosols have been characterized in terms of their mass for roughly two decades, and models have generally inferred their number concentrations indirectly from those mass estimates. But cloud droplet number concentrations, and therefore the brightness or albedo of marine clouds, depend on how many particles are available in the atmosphere rather than on how much material they collectively contain. A population of many small particles can seed far more cloud droplets than an equal mass concentrated in a few large ones. Without direct number measurements, the models were essentially guessing at the particle population that governs cloud formation over much of the planet.

Isolating the organic fraction from the rest of the sea spray required an elegant physical trick. At 90 percent relative humidity, sea salt and sulphate particles absorb water readily and grow substantially, whereas marine primary organic aerosol particles are considerably less hygroscopic and swell far less. The researchers exploited this contrast using a humidified tandem differential mobility analyser, an instrument that classifies particles according to how much they grow in humid air. Combined with concurrent measurements of particle size, number and chemical composition, this approach allowed the team to resolve the mPOA number size distribution down to an unprecedented 35 nanometres, a size range directly relevant to cloud droplet activation.

What emerged from the data was a distribution with two distinct modes rather than one. The measured size distribution is bimodal, with a smaller mode centred near 60 nanometres and a broader mode spanning roughly 100 to 200 nanometres. By contrast, the parameterization embedded in several major climate models produces a unimodal distribution centred at 120 nanometres, with no representation of the smaller mode at all. That omission is not a minor detail. Particles near 60 nanometres are precisely the sizes that can activate into cloud droplets at the moderate supersaturations typical of marine boundary layer clouds, so an entire population of potential cloud condensation nuclei was effectively invisible to the models.

The quantitative consequences are substantial. For particles larger than 35 nanometres, the model parameterization underestimates the total mPOA number concentration by approximately a factor of three. In the 30 to 70 nanometre range, the underestimate grows to between fivefold and sevenfold. Translated into cloud condensation nuclei, the mPOA contribution is underestimated by about 16 percent at a supersaturation of 0.25 percent and by as much as 64 percent at 0.5 percent supersaturation, assuming a 30 percent reduction in surface tension caused by the organic material. Surface tension matters here because organic films on particles can lower the energy barrier for droplet formation, allowing smaller particles to activate than would otherwise be possible. The affected models include GEOS-Chem, UKESM and CMAQ, which infer mPOA number from mass, as well as E3SM and CESM2, which represent mPOA without any size-resolved description.

To gauge what this means for the climate system, the researchers combined satellite observations of cloud properties with simulations from the GEOS-Chem model. As an order-of-magnitude estimate, the additional particles correspond to a first aerosol indirect effect of approximately -0.03 to -0.04 watts per square metre when averaged over the global ocean. Over biologically productive waters, where mPOA emissions are highest because phytoplankton activity is most intense, the effect is substantially larger, reaching -0.1 to -0.2 watts per square metre once the surface tension reduction is taken into account. These are cooling effects, since more cloud droplets produce brighter, more reflective clouds that bounce more sunlight back to space. While the global average figure may appear modest, it is concentrated over some of the most climatically sensitive ocean regions on Earth.

The authors are careful to frame these radiative estimates as sensitivity calculations rather than definitive global assessments. The figures quantify the magnitude of the mPOA influence that would obtain if the size distribution derived at Mace Head were representative of the global ocean, which is an assumption the data cannot yet confirm. Because the observations come from a single site, regional differences in phytoplankton community composition, sea state and atmospheric ageing may modify the size distribution elsewhere. Even so, the environmental conditions sampled at Mace Head were broad: chlorophyll-a concentrations ranged from approximately 0.1 to 10 milligrams per cubic metre and wind speeds from 5 to 20 metres per second, encompassing a substantial portion of the parameter space that characterizes the global ocean, although not all of it.

The implications extend well beyond correcting a single parameterization. Aerosol-cloud interactions remain among the largest sources of uncertainty in climate projections, and the total aerosol forcing that has masked part of greenhouse warming depends critically on getting natural aerosol backgrounds right. An underestimate of a natural aerosol source does not simply introduce a local error; it propagates into estimates of anthropogenic forcing, because the radiative effect of human-emitted particles is calculated relative to a preindustrial baseline in which marine organics played a role. If models have been undercounting the natural cloud-seeding particles over the oceans, the historical trajectory of aerosol cooling, and by extension the sensitivity of the climate system to greenhouse gases, may need refinement in the marine sector.

The findings also carry practical weight for emerging proposals to brighten marine clouds deliberately as a means of cooling the planet. Marine cloud brightening schemes depend on an accurate characterization of the natural marine aerosol baseline, since the effectiveness of adding particles to low-lying ocean clouds hinges on how many particles are already there and how close the cloud is to saturation with condensation nuclei. A baseline underestimated by a factor of three fundamentally changes those calculations. With the first direct size-resolved measurements now in hand, climate modelers have a concrete observational target to correct their parameterizations against, and atmospheric scientists have a clear agenda: extend such measurements to other ocean basins, different plankton communities and varying sea states, to determine how universal the bimodal signature observed over the North Atlantic truly is. The ocean’s influence on clouds, it turns out, is richer and more finely structured than the models assumed.

Subject of Research: Size-resolved measurement of marine primary organic aerosols and their role as cloud condensation nuclei in climate models

Article Title: Marine organic particles that seed ocean clouds may be three times more abundant than climate models assume

Article References: Marine organic particles that seed ocean clouds may be three times more abundant than climate models assume. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: marine primary organic aerosol, sea spray aerosol, cloud condensation nuclei, aerosol-cloud interactions, climate models, Mace Head, North Atlantic, bimodal size distribution, aerosol indirect effect, marine cloud brightening, phytoplankton, surface tension